Determining the location of a leak in a pipeline is important because it allows the operator of the monitored pipeline to take prompt and appropriate corrective action such as closing the proper pipeline isolation valves and dispatching of spill mitigation and pipeline repair crews to the proper locations.
Various methods have been used to find the location of leaks on pipelines. These methods may be classified as environmental monitoring methods and pipeline measurement-based methods.
Environmental monitoring methods use sensors external to the pipeline to detect the presence of the fluid contained within the pipeline. The existence of a leak is inferred from the presence of the fluid outside of the pipeline. The location is inferred to be near the sensor that detected the presence of the fluid.
Environmental monitoring methods have proven ineffective in detecting the leakage of some commonly used fluids since sensors are not available to differentiate the fluids of interest from those normally occurring in nature.
In addition to the problems in detecting the leakage of some fluids, it is often difficult to determine the location. For precision in determining the location, a large number of generally costly sensors are required. Various transient atmospheric and hydraulic conditions can influence the path a leaking fluid may take from the location of the leak to a sensor. Consequently, the assumption that the leak is closest to the sensor that detected it is not always valid.
Pipeline measurement-based methods can be categorized into two classes. The first class may be referred to as wave propagation time methods and the second may be referred to as energy grade line methods.
Wave propagation time methods detect the passage of the pressure expansion wave associated with a leak at two locations, one upstream of the leak and the other downstream. The location is determined by noting the exact time the expansion wave is detected by each of the two sensors and calculating the point of origin by considering, in addition to the detection times, the speed with which an expansion wave can propagate in the fluid of interest.
Wave propagation time methods have not worked well in actual applications because of several pratical limitations of the technique. The most significant problem is in detection of the expansion wave front itself. Special pressure transducers with a wide frequency response have been necessary to detect the wavefront and they are expensive and fragile. Often the changes in pressure that constitute the wavefront can be obscured by other changes in pressure that normally occur on the pipeline, thus rendering the pressure changes undetectable. As the expansion wave travels down a long pipeline its rise time decreases. This can cause a sensor close to the leak to detect its occurrence at an earlier time relative to the leading edge of the wavefront than does a sensor further away. This effect results in significant errors in the computed location of the leak.
Methods based on energy grade line calculations are based on the assumption that the energy grade line along a pipeline is of constant slope between ends when there is no leak.
When a leak occurs, a discontinuity in the slope of the energy grade line occurs at the location of the leak. This location may be found by projecting a new energy grade line based on inlet conditions from the inlet end of the line toward the leak and projecting a similar line based on outlet conditions from the outlet end of the line toward the leak. The point at which the lines intersect is the location of the leak.
Difficulties with this method have occurred in practice for at least two reasons. First, it is difficult to measure the pressure and flow accurately enough to get a precise location. Especially for small leaks, the two lines intersect at very shallow angles and consequently the computed location is very sensitive to small measurement errors. Second, the fundamental assumption that the leak is located at the point of discontinuity of slope of the energy grade line is true only when the line is flowing at a steady state. If the pipeline were initially at a steady state and a leak occurred, the pipeline would transition (typically over a period of minutes) to a new hydraulic load imposed by the leak. During this period of transition the assumptions on which the technique are based are not true since the line is not at a steady state. As a result the location process is very slow. It can take several minutes to an hour or more for the location process to be completed.
The following patents reflect the state of the art of which applicant is aware and are included herewith to discharge applicant's acknowledged duty to disclose relevant prior art. It is stipulated, however, that none of these references teach singly nor render obvious when considered in any conceivable combination the nexus of the instant invention as disclosed in greater detail hereinafter and as particularly claimed.
______________________________________ INVENTOR PATENT NO. ISSUE DATE ______________________________________ Kreiss 3,664,357 May 23, 1972 Matthews, Jr. et al. 3,903,730 September 9, 1975 Covington et al. 4,012,944 March 22, 1977 Hirano 4,090,179 May 16, 1978 Covington et al. 4,091,658 May 30, 1978 Elliott et al. 4,106,099 August 8, 1978 Covington et al. 4,144,743 March 20, 1979 Covington 4,308,746 January 5, 1982 Anway B 1 4,083,229 February 1, 1983 Hogan 4,375,763 March 8, 1983 Anway 4,083,229 April 11, 1978 Burgess et al. 4,384,475 May 24, 1983 Hogan 4,402,213 September 6, 1983 Claude 4,450,711 May 29, 1984 Werner 4,507,128 March 26, 1985 Anthony et al. 4,574,618 March 11, 1986 Cota et al. 4,586,142 April 29, 1986 Barkhoudarian 4,612,797 September 23, 1986 Schwarz et al. 4,621,520 November 11, 1986 Holm et al. 4,625,545 December 2, 1986 Sugimoto et al. 4,650,636 March 17, 1987 Farmer 4,796,466 January 10, 1989 ______________________________________
The patents to Covington are interesting in that they reveal a method and apparatus for leak detection based on monitoring the characteristics of the fluid within a pipeline. The Covington patents are distinguishable from the instant invention, however, in that the instant invention provides both leak detection and leak location. The Covington patents provide only leak detection.
The patent to Burgess et al. describes a device capable of detecting and locating leaks in a fluid-filled cable. This device relies on measurements of the fluid contained to determine the location of the leak. This invention requires that the fluid be static and also requires valves to be opened and closed while fluid measurements are taken. The invention of this application relies on fluid measurements in a flowing fluid and requires no altering of the fluid flow through adjustment of valves to determine the leak's location. Thus, the leak location device of this application can operate without a human operator overseeing the leak location process and can occur when the fluid is in motion.
The patent to Anway is also of interest in that it is capable of locating and detecting a leak in a fluid pipeline. The Anway invention uses a vibration detector which detects the vibrations associated with a leak. The invention of this application is distinguishable in that it relies on measurements taken from the fluid itself as opposed to vibrational measurements from the pipeline.
The patent to Hogan is capable of determining the location of a leak in a fluid-filled pipeline. The Hogan invention requires that an apparatus be passed through the interior of the pipeline to determine the leak's location. The invention of this application is capable of leak location without requiring that any apparatus be placed within the pipeline.
The remaining prior art references diverge even more starkly from the instant invention than those described particularly hereinabove.